A lithographic printing plate widely used at present is generally prepared by applying printing plate-making treatments such as image exposure, development, washing, lacquering, etc., to a so-called "presensitized printing plate (PS plate)" prepared by coating a photo-sensitive composition on an aluminum plate the surface of which is subjected to a graining treatment. By image exposure, a difference in solubility for a developer in the subsequent development is formed between the exposed areas and the unexposed areas of the layer of the foregoing photo-sensitive composition and then either the exposed areas or unexposed areas of the photosensitive composition layer are dissolved off or peeled off by the subsequent development process while leaving other areas of the layer on the aluminum plate to form an image. The image areas, i.e., the areas composed of the remaining photosensitive composition layer are ink receptive while in the non-image areas the hydrophilic aluminum support surface is exposed by the removal of the photosensitive composition layer as described above and hence the non-image areas are water receptive.
Accordingly, a dampening or fountain solution (water or an aqueous solution) is supplied to the surface of the plate having both the foregoing areas, whereby the film of the dampening solution is retained at the non-image areas, an ink is applied to the image areas, and by repeating the step of transferring the ink applied to the image areas directly or indirectly (i.e., after temporarily transferring the ink onto a blanket) onto the surface of a paper, printing is performed.
In such a printing system, by suitably selecting a photosensitive composition coated on a support, it is possible to obtain a printing plate capable of providing more than 100,000 good copies from one printing plate. However, in the present situation of printing industries, a printing plate capable of printing further increased copies is demanded and further by the reasons of the reduction of the cost for printing plate and simplification of printing operation, it has been keenly desired to obtain more copies by the same printing plate, i.e., to improve the press life of a printing plate.
With a lithographic printing plate using aluminum as the support, a so-called burning-in process is employed as a means of improving the press life. That is, a method of heating, to a high temperature, a lithographic printing plate prepared by image exposing and developing a photosensitive printing plate by ordinary manners, whereby the composition forming the image areas is hardened by heat to strengthen the image areas is effective and has generally been employed. In this case the heating temperature and the heating time for the burning-in process usually depend upon the composition forming the image but are generally about 200.degree. to 280.degree. C. and about 3 to 7 minutes, respectively. However, even in the burning-in process, it has been desired to perform the process at a higher temperature and for a shorter operation time for further improving the press life of the printing plate and for further shortening the operation time.
On the other hand, an aluminum plate used as a support for lithographic printing plates is prepared by subjecting an aluminum alloy such as AA1050, AA1100, AA3003, etc., to an ordinary continuous casting to form an ingot of the aluminum alloy, applying hot rolling, cold rolling, and, if necessary, heat treatment in processing the ingot of the aluminum alloy to form a plate or a web of the aluminum alloy, graining the surface thereof a mechanical method, a chemical method, an electrochemical method or by a composition of these methods, and further, if desired, anodically oxidizing the grained plate or web.
However, an aluminum alloy plate composed of the aluminum alloy as described above conventionally used for the aforesaid purpose causes a recrystallization phenomenon of aluminum when the aluminum alloy plate is heated to a temperature of higher than 280.degree. C., thereby greatly reducing the strength of the plate and losing firmness of the plate. Therefore, when such an aluminum plate is used as a support for lithographic printing plates, there are problems in that handling of the printing plate is very difficult. For example, it becomes impossible to mount the printing plate on a printing cylinder of a press, and it is impossible to make resistering the printing plate in the case of multicolor printing.